Subject and object are the dual objective faces of what subjectively they are - sometimes absurdly and misleadingly described as the 'Middle Way'.
What subjectively they are can only be known as Void because the knowing of that is an attempted objectivisation of what-they-are, whereby nothing can be cognised - since what is cognising cannot cognise itself.
Void, however, is not such - for it is I. What they are is what we are and what, for every sentient being, is what I am.
And I am the presence of the absence of all that seems to be.
This formulation applies to all pairs of 'opposites', for instance all that is conceived as 'obverse or reverse', 'heads or tails', front or back', 'here or there', 'this or that', 'pile ou face'. Phenomenally regarded, they are mutually exclusive alternatives, one or the other.
'Self or other', 'noumenon or phenomenon' are not different, since all as such are objects - even 'subject' and 'noumenon' - conceptually regarded. But if 'and' is substituted for 'or', or if the nouns are hyphenated, as 'subject-object', each pair is then being regarded as one single object - which they can never be positively but only as a result of their mutual negation, which requires not 'and' but 'neither ... nor ...'
Always 'noumenon or phenomenon' are the essential pair of opposites, for the one implies the source of all that could be, and the other defines every thing that could appear. They are, therefore, all-inclusive.
Regarded objectively, which is equivalent to being regarded at all, they are the negative and positive faces of what subjectively they are - like any other pair of 'contraries', since both are then being objectified. Thus what they are is their mutual negation or the absence of no-phenomena. But they are then and also what we are, what each of us is as 'I'. Phenomena are what we appear to be as a result of an interpretation of sensorial perception, and noumenon is what each of us is antecedent to this perceptive-conceptive process, manifested and unmanifested respectively.
This, of course, can only be suggested by the personal pronoun 'I'. But whereas, on the one hand, 'I' as a noun implies either noumenality or phenomenality, on the other, as a pronoun, it implies neither the one nor the other; devoid of any qualification soever, it implies the origin of the origin of phenomena, the origin of this by which phenomena are manifest, and in which both entirely inhere in subjectivisation.
No word, or form of words, no sound or symbol could ever indicate the meaning of 'I', which Maharshi called 'I-I', but even this Sanscrit locution, though it could hardly be bettered, is inadmissible. Why? Simply because the mere attempt to express, and so objectify it, is turning away from what it is - which is also the turning that is turning away from what is turning away from turning.